WALKER, Sears Cook, American mathe matician and astronomer: b. Wilmington, Mid dlesex County, Mass., 28 March 18A5; d. Cin cinnati, Ohio, 30 Jan. 1853. He was graduated at Harvard College in 1824, taught school near Boston for two years and in 1827 removed to Philadelphia, where also he engaged in teach ing. His parallactic tables, first prepared in 1834 for the latitude of Philadelphia, reduced the time needed for computing the phases of an occultation to less than half an hour. In 1837 he was invited to prepare a plan for the or ganization of an observatory in connection with the Philadelphia High School, and from its equipment in 1840 until 1852 he published in the 'Proceedings of the American Philosophical Society' and the American Journal of Science frequent and copious observations and investi gations which he had made. In 1841 he pub lished a valuable memoir on the periodical meteors of August and Novetnber. In 1845 he took part in the Washington naval observa tory, where on 2 Feb: 1847, four months after the detection of the planet Neptune, he made the discovery that a star observed by Lalande in May 1795 must in fact have been this planet. By subsequent alternating computa
tions of Pierce and Walker, the former in vestigating the perturbations and the latter the orbit, the theory of Neptune was at once placed on a footing comparable with that of the other large planets. In 1847 he was invited to take charge of the longitude computations of the United States Coast Survey, an office in which he continued until his last illness. By the joint labors of Walker and Bache the method of telegraphic longitude determinations was de veloped and successfully carried out as early as 1849, with greater precision than was at tained in Europe 10 years later. The intro duction of the chronographic method of re cording observations belongs to Walker and Bache. The prosecution of the telegraphic method of longitude soon led Walker to the discovery that the time required for the trans mission of the galvanic signal was measurable, and the velocity by no means as high as had been supposed.